The modulation of galactic cosmic rays in the interplanetary medium

Abstract

The anisotropic diffusion of cosmic rays in the interplanetary medium is discussed with the assumption that the fluctuating component of the magnetic field produces a scattering effect which can be described by a simple mean free path treatment. The results of this discussion are applied to a plausible model of the interplanetary medium with the aim of providing a basis for the understanding of the processes causing the more important modulations of the galactic cosmic ray intensity. It is suggested that a gradient of the cosmic ray density exists, and that this plays a significant role in producing the solar cycle and 27-day modulation effects; in addition, the associated cosmic ray pressure gradient might affect the size of the cavity resulting from the interaction between the solar wind and the interstellar medium. Since the gradient could be present even at sunspot minimum it is possible that the energy density of cosmic rays in the galaxy is larger than the maximum value observed at the Earth, and hence this value should be regarded only as a lower limit. The main contribution to the solar diurnal variation of the cosmic ray intensity is suggested to be due to co-rotation of the cosmic rays with the Sun. However above a certain energy, which varies through the solar cycle, the galactic cosmic rays appear to be unaffected by the roughness of the interplanetary magnetic field, thus any streaming which occurs at these higher energies should cause a sidereal diurnal variation of the intensity.